A combination drill-reamer is reground as one tool, but it is designed as two. The reaming stage sets the bore size, the drill stage sets the stock that reaches it, and the transition between them fixes the relationship. Regrinding moves all three at once.
That coupling is why the number of regrinds is a geometric result rather than a supplier promise. This page sets out the three limits, the way the drill section reacts when the reaming stage loses diameter, and the cost per hole at each regrind count.
Three limits decide the count: the stock left for reaming as the reaming diameter falls, the step height across the transition, and the flute depth in the drill section. On the example tool the first limit is reached after two regrinds.
The cost per hole keeps falling with each regrind, from 0.120 to 0.073 EUR on the example, which is exactly why the geometric limit has to be checked before the cost table is used.
Every dimension and price below is an example, chosen so the arithmetic can be repeated. No regrind count is promised for any HENTRY tool on this page; the count follows from the drawing and the pre-hole.
Why a combination tool is harder to regrind
A single-purpose reamer is reground on one diameter and one lead. A combination tool is reground on two cutting zones that share a centreline, a body and, in most designs, a single grinding datum.
When the reaming stage is reground, its diameter drops. When the drill stage is reground, its length drops. The transition has to be re-established after both, and it cannot move without changing the axial position of one stage relative to the other.
That is the coupling: a change made to restore the bore size alters the stock that feeds it, and a change made to restore the drill length alters where the reaming stage starts.
On a tool with a fixed transition, the grinder has three choices and all of them are visible on the part: regrind the reaming stage only and accept a smaller allowance, regrind both and accept a shorter drill, or re-datum the transition and accept a new grinding setup.
What this means commercially is simple. The regrind price is not the constraint; the number of times the geometry allows a regrind is. A supplier quoting eight regrinds for a tool with a 0.20 mm diametral allowance is quoting a number that the drawing will not support.
The three limits on regrind count
The three limits are independent, and the count is set by whichever is reached first. On most combination tools the allowance limit comes first, which is why it is listed first.
The allowance limit is the stock left between the pre-hole and the reaming diameter. Each regrind removes a fixed amount from the reaming diameter; the tool stops being usable when the remaining stock per side falls below the minimum the reaming stage needs to cut rather than rub.
The step limit is the height between the drill stage and the reaming stage. Regrinding changes that height unless the transition is re-cut, and the drawing usually gives it a tolerance of a few hundredths of a millimetre.
The chip-space limit is the flute depth in the drill section. Regrinding the drill reduces the flute depth, and with it the volume available for the chip at the feed rate the tool was designed for.
| Limit | Where it comes from | How to judge it |
|---|---|---|
| Stock left for reaming | Reaming diameter minus pre-hole diameter, divided by two | Measure the pre-hole and the reground reaming diameter; stop when the remaining stock per side falls below the minimum for the material |
| Step height across the transition | Axial distance between the drill stage and the reaming stage | Compare with the drawing tolerance after each regrind; re-datum the transition when the cumulative shift reaches the band |
| Flute depth in the drill section | Chip volume required at the design feed | Check the remaining flute depth against the chip area for the feed and material; stop when the depth falls below it |
Work the example. The pre-hole is 11.80 mm, the reaming stage is ground to 12.012 mm, so the diametral stock is 0.212 mm, or 0.106 mm per side. A minimum of 0.05 mm per side is required to cut rather than rub.
Each regrind removes 0.06 mm from the reaming diameter, or 0.03 mm per side. After one regrind the stock is 0.076 mm per side, after two it is 0.046 mm per side, which is below the minimum. The allowance limit is therefore two regrinds on this pre-hole.
The step limit and the chip-space limit are checked the same way, with the drawing tolerance and the design feed. On the example both allow three regrinds, so the allowance limit governs and the answer is two.
The coupling effect on the drill section
When the reaming stage loses 0.12 mm of diameter over two regrinds, the drill stage has not changed. The stock reaching the reamer has therefore fallen by 0.12 mm, and the reamer is being asked to remove less material at the same feed.
Less stock is not automatically better. Below a minimum, the reaming edges rub instead of cut, which raises temperature, smears the surface and produces a bore that measures on size but fails on roundness or surface finish.
There are two ways to restore the allowance, and both have consequences. Opening the pre-hole restores the stock but changes the drill stage's job, and on a combination tool the pre-hole is made by the same tool, so the drill stage has to be reground to cut larger.
Regrinding the drill stage to a larger diameter shortens its life in a different way: the margin widens, the flute depth falls, and the chip space shrinks. This is the mechanism behind the third limit and the reason the two stages cannot be optimised separately.
The alternative is to accept a smaller allowance and change the reaming parameters. That works within limits, but it moves the process away from the conditions the tool was designed for, so it should be a documented decision rather than a shop-floor adjustment.
Whichever route is taken, the allowance has to be rechecked after every regrind. The method for that check is the same one used to set the pre-hole in the first place, set out in reaming allowance and pre-hole size.
Regrind count versus cost per hole
The cost per hole falls with each regrind, because the fixed purchase price is spread over more holes. That is the argument for regrinding, and it is also the argument that has to be checked against the geometric limits.
The example tool costs 360 EUR. A regrind with recoating costs 125 EUR and restores about 90 percent of the original life of 3,000 holes, so each regrind adds 2,700 holes.
| Regrinds used | Cost per hole (EUR) | Cumulative holes |
|---|---|---|
| 0 | 0.120 | 3,000 |
| 1 | 0.085 | 5,700 |
| 2 | 0.073 | 8,400 |
| 3, where the geometry allows | 0.066 | 11,100 |
Check the arithmetic: 360 divided by 3,000 is 0.120. After one regrind the accumulated cost is 485 EUR over 5,700 holes, which is 0.085. After two it is 610 EUR over 8,400 holes, which is 0.073.
The table says the third regrind is the cheapest per hole. The geometry says the third regrind is not available on the example tool. Both statements are true, and the second one decides what can be quoted.
Two costs are missing from the table and both push the answer up. Re-qualification after a regrind, usually a first article plus a capability check, is a fixed cost per event. Scrap caused by a reground tool that no longer cuts cleanly is a variable cost per hole.
Add them and the curve flattens. On the example, one hour of re-qualification at 45 EUR per regrind adds about 0.008 EUR per hole at the two-regrind level, which is small. A scrap rate of 0.5 percent at 40 EUR per part adds about 0.20 EUR per hole, which is not.
When to stop
There are three stop signals, and they do not arrive at the same time. The first is geometric and can be calculated before the tool is used; the other two are measured in production.
The geometric stop is the allowance limit above. It is known from the drawing, the pre-hole and the amount removed per regrind, and it should be written on the tool card before the first regrind, not discovered at the third.
The wear stop is the wear band on the reaming edges. When the band reaches the width that experience sets for the material, the tool is reground whether or not the bore has moved. Waiting for the bore to move means the process has already produced parts at the edge of tolerance.
The trend stop is a bore size that drifts in one direction across a run. A drift is a signal about the tool or the process, and regrinding without understanding it usually moves the problem to the next tool.
Control-chart logic applies here, which is why the sources below include the NIST page on control charts.
The condition stop is damage: chipping on the PCD corners, a cracked transition, or a worn margin on the drill stage. Damage is not a regrind decision; it is a scrap decision, and the tool should be assessed before it is sent for regrinding.
The regrind criteria for single-purpose PCD reamers are set out in regrinding and reconditioning of PCD reamers. The combination tool inherits those criteria and adds the coupling described on this page.
What a regrind record should contain
A regrind record is the only way to know where a tool sits in its life. Without one, the regrind count is a guess and the cost per hole is an average over tools in different conditions.
Record the reaming diameter before and after each regrind, the pre-hole diameter measured on the same day, the amount removed, the step height, and the number of holes since the previous regrind. Five numbers per event are enough.
Record the reason as well: wear band, bore drift, damage or scheduled interval. The reason is what turns a list of numbers into a decision about the next tool.
Record the outcome after regrinding: the first-article result, the bore size at the start of the run and the holes produced before the next regrind. Those three fields show whether the regrind restored the tool or merely postponed its replacement.
Keep the record with the tool, not only in the purchasing system. The person who decides whether to regrind a fourth time is standing at the machine, and the drawing tolerance has to be in front of them.
Send the record with a regrind request through the custom cutting tool RFQ. A tool with a history can be assessed before it is shipped; a tool without one has to be inspected first, which costs a week.
Two related pages carry the rest of the decision. The bore-quality cost of a combination tool is set out in hole quality tradeoffs for combination tools, and the upstream question of whether to combine the operations at all is in when to use a combination drill-reamer.
Wear criteria are the input to the regrind count: the call on whether the edge is still serviceable is in wear criteria for combination drill-reamers.
What published sources say about this
These pages were used for the refurbishment, wear and geometry points. Each is cited for what that page states, under its own conditions.
- Tool refurbishment (toolmaker service page): regrinding and recoating to original geometry and coating, with the statement that regrinding costs a fraction of a new tool and extends tool life, the commercial basis for the cost table.
- Digitalised regrinding management (toolmaker report): reconditioning described as a significant expense whose registration and turnaround are the critical parts, which is the argument for the regrind record above.
- c-Com wear detection (toolmaker application page): wear identified from a photographed cutting edge with recommended actions, the practical version of the wear stop described here.
- Reamers, machine reamers (toolmaker technical overview): reamer structure, guide part and flute shapes, the basis for treating the reaming stage as a geometry that loses size with each regrind.
- Reaming and fine boring (toolmaker technical page): reaming tool designs, guide pads and floating holders, the designs in which the transition and the guide part interact most strongly with regrinding.
- Dimensioning and Tolerancing (ASME Y14.5): the runout, concentricity and profile definitions that decide whether a reground two-stage tool still meets its drawing.
- What are control charts? (NIST/SEMATECH e-Handbook of Statistical Methods): the logic of plotting a measured characteristic in time order, the method behind the bore-drift stop signal.
Outbound references are listed because the geometry and cost arguments depend on them, not as endorsements. The example dimensions and prices are HENTRY's illustration and are not taken from these pages.
Frequently asked questions
How many regrinds can a combination drill-reamer take?
There is no general number. On the example tool the allowance limit allows two regrinds, and a tool with a larger diametral allowance or an adjustable pre-hole can take more.
The count is calculated from the drawing, the pre-hole and the amount removed per regrind, and it should be written on the tool card before the first regrind.
Does regrinding the reaming stage change the drill stage?
Not directly, but it changes the stock that reaches the reaming stage. A smaller reaming diameter with an unchanged pre-hole means less material to remove, and below a minimum the edges rub instead of cut.
Restoring the stock means opening the pre-hole, which on a combination tool means changing the drill stage as well.
Why not always use the maximum number of regrinds?
Because the cost per hole is not the only number that matters. Re-qualification, scrap risk and the shrinking process window all rise with the regrind count.
On the example the third regrind is the cheapest per hole and is not geometrically available, which is the simplest illustration of the difference between the two questions.
Can a damaged tool be reground?
Sometimes, and the assessment has to happen before the tool is shipped. A chipped PCD corner can often be removed; a cracked transition or a damaged body usually cannot.
Send the tool history and a photograph of the damage, and the assessment can be made without shipping the tool twice.
What should trigger a regrind instead of a replacement?
A wear band at the width set for the material, a bore that has started to drift, or a scheduled interval that the records support.
Replacement is the right answer when the remaining allowance is below the minimum, when the step height can no longer be held, or when the flute depth no longer carries the chip at the design feed.
Updated 13 November 2026: first publication. Scope: why a two-stage tool is harder to regrind, the three limits on regrind count, the coupling between reaming diameter and drill stage, the cost per hole and the stop signals. All dimensions and prices are examples, not HENTRY values.